CVE-2026-123817
Description
A flaw was found in aiohttp. When reassembling incomplete WebSocket frames, the reader allocates separate memory objects for each incoming data fragment without enforcing an upper limit on the object count. A remote attacker, acting as a malicious client or server, can exploit this vulnerability by transmitting a frame split into numerous tiny segments. This behavior generates excessive memory overhead that bypasses configured message size thresholds, leading to a Denial of Service (DoS) from resource exhaustion.
Statement
This vulnerability is rated as an Important severity flaw because an unauthenticated remote attacker can trigger severe memory exhaustion and crash the hosting process without requiring special privileges. In Red Hat environments running Python applications based on aiohttp, exposed WebSocket servers and client connections are vulnerable to crafted message streams split across tiny network reads. Because transport overhead accumulates independently of application message size limits, memory consumption can quickly exhaust available system resources and disrupt collocated workloads.
Mitigation
Restrict network exposure of applications utilizing aiohttp WebSocket endpoints to localhost or trusted networks using firewall rules.
1. If the application only requires local communication, configure the service to bind solely to loopback interfaces (such as 127.0.0.1 or ::1).
2. For services requiring network access, restrict reachability using firewalld:
firewall-cmd --permanent --zone=trusted --add-source=<trusted_ip_or_network>
firewall-cmd --permanent --zone=public --remove-port=<application_port>/tcp
firewall-cmd --reload
Caveats: Limiting network reachability blocks unauthorized external connections and will prevent external clients from accessing the WebSocket service.
Warning: Modifying firewall rules and reloading firewalld may briefly interrupt active network connections, and restarting application services to change listening addresses will disrupt active client sessions.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | N/A | N/A |
| Attack Vector | Network | N/A | N/A |
| Attack Complexity | Low | N/A | N/A |
| Privileges Required | None | N/A | N/A |
| User Interaction | None | N/A | N/A |
| Scope | Unchanged | N/A | N/A |
| Confidentiality | None | N/A | N/A |
| Integrity Impact | None | N/A | N/A |
| Availability Impact | High | N/A | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
When allocating resources without limits, an attacker could prevent other systems, applications, or processes from accessing the same type of resource. It can be easy for an attacker to consume many resources by rapidly making many requests or causing larger resources to be used than is needed.
Frequently Asked Questions
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